Electronic devices have been elaborated which are fabricated by forming superconducting, dielectric or ferroelectric films on silicon substrates or semiconductor crystal substrates, followed by integration. Attempts have been made to fabricate advanced devices by combining semiconductors with superconductors, dielectrics or ferroelectrics. For example, for the combination of semiconductors with superconductors, SQUID, Josephson devices, superconducting transistors, electromagnetic wave sensors and superconductor wired LSI are contemplated. For the combination of semiconductors with dielectric materials, LSIs having a higher degree of integration and dielectric isolated LSIs relying on SOI technology are contemplated. For the combination of semiconductors with ferroelectric materials, non-volatile memories, infrared sensors, optical modulators, optical switches and OEIC are contemplated.
To ensure optimum device characteristics and reproducibility thereof for these electronic devices, the superconducting, dielectric and ferroelectric materials used must be single crystals. With polycrystalline materials, it is difficult to provide good device characteristics on account of the disturbances of physical quantities by grain boundaries. The same applies to thin film materials. Epitaxial films which are as close to complete single crystals as possible are desired.
Aiming at the above application, engineers have made investigations on epitaxial films. Ferroelectric epitaxial films on MgO substrates are described in J. Appl. Phys., 76 (12), 15, 7833 (1994), for example.
It must be possible to integrate a semiconductor with a ferroelectric material before the technology can be applied to practical devices, although it is extremely difficult to incorporate a MgO substrate in a silicon device. It is also quite difficult to form a unidirectionally oriented ferroelectric thin film on a silicon single crystal substrate, for example, to form a (001) unidirectionally oriented BaTiO.sub.3 film with good crystallinity on a silicon (100) substrate. In this regard, we proposed in Japanese Patent Application No. 217884/1996 a process for preparing an epitaxial thin film of ferroelectric material on a silicon single crystal substrate.
In general, when a thin film of ferroelectric material is formed on a silicon substrate, the properties of this ferroelectric thin film are significantly inferior to those estimated from the ferroelectric material itself. The properties of ferroelectric material, for example, dielectric constant, Curie temperature, coercive field, and remanent polarization, vary with the stress the ferroelectric material possesses. Since a thin film of ferroelectric material has a likelihood that stresses are produced during film formation, stress control becomes crucial in order to form a satisfactory ferroelectric thin film. The deterioration of properties of ferroelectric thin films on silicon substrates is largely attributable to the influence of stresses.
In J. Appl. Phys., 76 (12), 15, 7833 (1994) and Appl. Phys. Lett., 59 (20), 11, 2524 (1991), for example, it is reported that two-dimensional stresses within a film plane have a significant influence on ferroelectric properties although use is made of MgO single crystal substrates rather than silicon single crystal substrates. The main cause of stress generation is the difference in physical properties (e.g., coefficient of thermal expansion and lattice constant) between the underlying substrate and the ferroelectric material. Therefore, in order to apply ferroelectric thin films to devices, the stress must be controlled; otherwise, desirable ferroelectric properties are not consistently obtained.
Among ferroelectric materials having favorable properties, there are known lead family ferroelectric materials such as PbTiO.sub.3, PLT (PbTiO.sub.3 having La added thereto), PZT (PbZrO.sub.3 -PbTiO.sub.3 solid solution), and PLZT (PbZrO.sub.3 -PbTiO.sub.3 solid solution having La added thereto). Since most of the lead family ferroelectric materials have an axis of polarization in [001] direction, (001) unidirectionally oriented films are preferable from the standpoint of ferroelectric properties. However, we found through research works that when a lead family ferroelectric material thin film is formed on a silicon single crystal substrate, there is often formed a domain structure in which (001) oriented crystals and (100) oriented crystals are in admixture. By adjusting the composition, for example, by increasing the amount of La added in the case of PLT thin film, it is possible to avoid the frequent formation of the domain structure, but at the sacrifice of ferroelectric properties. Also in the case of PZT thin film which is often used as the superior ferroelectric material, the domain structure is likely to develop in the composition region where good ferroelectric properties appear, that is, where the atomic ratio of Ti/Zr is relatively high, rendering it quite difficult to form a unidirectionally oriented film. Thin films with such a domain structure have significantly lower ferroelectric properties than single crystals and also than when formed on MgO substrates.
For the following reason, it is difficult to form a unidirectionally oriented thin film of lead family ferroelectric material on a silicon single crystal substrate. In the following description, reference is made to PZT as a typical example of the lead family ferroelectric material. Both silicon and MgO have lower coefficients of thermal expansion than PZT. In particular, silicon has a coefficient of thermal expansion of 2.6.times.10.sup.-6/ .degree. C. which is significantly lower than that (14.times.10.sup.-6 /.degree. C.) of MgO. Then, provided that a PZT thin film is formed at a temperature of 600.degree. C., for example, the silicon substrate inhibits the PZT thin film from contracting in the course of cooling to room temperature after the film formation, causing a relatively high two-dimensional tensile stress to be produced in the PZT thin film within its plane. The film having such a high two-dimensional tensile stress undergoes a drop of spontaneous polarization. Tending to mitigate the tensile stress, the PZT film becomes a film of a mixture of (001) and (100) oriented crystals, which results in a significant drop of spontaneous polarization. Even if the PZT thin film becomes a (001) unidirectionally oriented film, the ferroelectric properties of this PZT thin film are inferior to those of a film of a (001) and (100) orientation mixture because of the presence of a substantial tensile stress in the PZT thin film.
In the recent years, as described in Appl. Phys. Lett., 68, 2358 (1996), research efforts have been made on ultrahigh density recording media wherein information is recorded by reversing the polarization of a ferroelectric material by means of an atomic force microscope (AFM) probe or the like.
The ferroelectric material undergoes polarization reversal at a certain threshold voltage. When such a ferroelectric material is used as a recording film, information is written therein by applying a pulse voltage to the ferroelectric film by means of an AFM probe or STM (scanning tunnel microscope) probe for aligning the polarization of only the probed region in one direction or reversing the polarization. For information read-out, the piezoelectric effect, pyroelectric effect, electro-optical effect and current detection upon polarization reversal of the ferroelectric material can be utilized.
The AFM or STM has a resolution of the atomic level. The ferroelectric material incurs polarization reversal at a high speed of 100 ns or less and permits record bits to be formed to a diameter of 10 nm or less. Then a high density memory with a capacity of about 10.sup.6 megabits/cm.sup.2 can be realized if record bits are formed as zones of 10 nm.times.10 nm, for example.
As the aforementioned ferroelectric media for AFM or STM memories, structures having a conductive layer formed on a substrate and a polycrystalline ferroelectric thin film formed thereon have been used. In the media using a polycrystalline ferroelectric thin film, crystal grain boundaries, domains and surface irregularities of the ferroelectric thin film produce noise.
Since the distance between the probe and the medium in the AFM or STM memory is of the order of nanometers, the memory medium is also required to have a surface which is flat or uniform on the order of nanometers. Uniformity is important with respect to surface irregularities, crystallinity, domains, and surface-trapped charges. Since the position of the probe relative to the recording medium is controlled by means of an actuator when addressing a record bit for writing and reading, the correct reading of the relevant bit is prohibited or high speed operation is prevented if the uniformity with respect to surface topography, crystallinity, domains, and surface-trapped charges is poor. If the flatness of the medium is poor, the noise caused by surface irregularities, crystallinity, domains, and surface-trapped charges is contained into the record bit signal. The prior art media using ferroelectric thin films are insufficient in surface uniformity, and media having a surface which is uniform on the order of nanometers have not been realized.
From the standpoint of realizing a fully flat recording medium, flatness on the molecular level is accomplished if a ferroelectric material is formed on a substrate as an epitaxial film. In preparing this ferroelectric medium, however, a ferroelectric film must be formed on a substrate having a conductive thin film already formed thereon. On the conductive thin film having poor flatness, a ferroelectric thin film having a fully flat surface cannot be formed. In order that the ferroelectric material be formed in the form of an epitaxial film, the conductive thin film must have a crystal lattice matched with the ferroelectric material.
Moreover, as previously described, the ferroelectric material used herein should be formed on silicon substrates for practical application, and its ferroelectric properties should not be deteriorated by stresses, domain formation or the like. Therefore, for ultra-high density recording media of the type that information is recorded by inducing polarization reversal in the ferroelectric material by means of an AFM probe, not only a conductive thin film having a crystal lattice matched with the ferroelectric material, high crystallinity and high surface flatness, but also a ferroelectric thin film having excellent ferroelectric properties which are not deteriorated by stresses, domain formation or the like are necessary. Additionally, the conductive thin film and the ferroelectric thin film of the above-mentioned quality must be realized on silicon substrates which are practical substrates. However, film structures having such thin films have never been available.
It is disclosed in Japanese Patent Application No. 245642/1996 by TDK Corporation, for example, that a ferroelectric film having improved flatness is obtained by depositing a ferroelectric material on a conductive thin film of platinum. The ferroelectric thin film formed on the platinum thin film has satisfactory flatness in relatively short periodicity, but a further improvement in flatness in relatively long periodicity is needed.
As mentioned above, when a ferroelectric thin film, especially PZT thin film is formed on a silicon single crystal substrate, the film does not have a sufficient spontaneous polarization because a substantial two-dimensional tensile stress is left within film plane. Further, a flat conductive thin film cannot be formed on the silicon single crystal substrate.